In the search for high performance materials, considerable interest has been focused upon carbon fibers. The terms "carbon" fibers or "carbonaceous" fibers are used herein in the generic sense and include graphite fibers as well as amorphous carbon fibers. Graphite fibers are defined herein as fibers which consist essentially of carbon and have a predominant x-ray diffraction pattern characteristic of graphite. Amorphous carbon fibers, on the other hand are defined as fibers in which the bulk of the fiber weight can be attributed to carbon and which exhibit an essentially amorphous x-ray diffraction pattern.
Industrial high performance materials of the future are projected to make substantial utilization of fiber reinforced composites, and carbon fibers theoretically have among the best properties of any fiber for use as high strength reinforcement. Among these desirable properties are corrosion and high temperature resistance, low density, high tensile strength and high modulus. During such service, the carbon fibers commonly are positioned within the continuous phase of a resinous matrix (e.g. a solid cured epoxy resin). Uses for carbon fiber reinforced composites include aerospace structural components, rocket motor casings, deep-submergence vessels, ablative materials for heat shields on re-entry vehicles, strong lightweight sports equipment, etc.
As is well known in the art, numerous processes have heretofore been proposed for the thermal conversion of organic polymeric fibrous materials (e.g. an acrylic multifilamentary tow) to a carbonaceous form while retaining the original fibrous configuration substantially intact. During commonly practiced carbon fiber formation techniques, a multifilamentary tow of substantially parallel or columnized carbon fibers is formed with the individual "rod-like" fibers lying in a closely disposed side-by-side relationship. See for instance, the following commonly assigned U.S. Pat. Nos. 3,539,295; 3,656,904; 3,723,157; 3,723,605; 3,775,520; 3,818,082; 3,844,822; 3,900,556; 3,914,393; 3,925,524; 3,954,950; and 4,020,273.
Recently, there has been interest in the use of ceramic materials, including ceramic fibers for a number of high temperature, high performance applications such as gas turbines. These applications require a unique combination of properties such as high specific strength, high temperature mechanical property retention, low thermal and electrical conductivity, hardness and wear resistance, and chemical inertness.
Among the ceramic materials which have been suggested are those made from organosilicon polymers. Thus, polymers based on silicon, carbon and/or nitrogen and oxygen have been developed. See, for example, "Siloxanes, Silanes and Silazanes and the Preparation of Ceramics and Glasses" by Wills et al, and "Special Heat-Resisting Materials from Organometallic Polymers" by Yajima, in Ceramic Bulletin, Vol. 62, No. 8, pp. 893-915 (1983), and the references cited therein.
Other metallic polymers can be formed into ceramic materials. Thus, U.S. Pat. No. 4,581,461 forms boron nitride by pyrolyzing B-triamino-N-tris (trialkylsilyl)borazines. The formation of aluminum nitride fibers is disclosed in commonly assigned, copending application Ser. No. 872,312, filed June 9, 1986. Aluminum nitride ceramics are formed by thermal conversion of poly-N-alkyliminoalanes. Ceramics comprising silicon carbide and aluminum nitride solid solutions are also disclosed. These ceramic alloys are formed by thermal conversion of a mixture of an organosilicon preceramic polymer and the above-mentioned aluminum-containing polymer. Moreover, many recent patents describe specific silicon-containing preceramic polymers which are formed into silicon carbide and/or nitride upon thermal treatment.
Another technique which has been suggested for producing ceramic fibers such as metal carbide fibers has involved incorporating metallic additives into a carbon fiber product, the precarbonaceous polymer forming solution, the polymer spinning solution or the polymer fiber subsequent to spinning, and converting the metallic compounds in situ to metal carbides upon thermal conversion. In these methods the precarbonaceous polymer acts as the source of carbon.
An important ceramic fiber formed by such method is boron carbide and boron carbide-containing carbon fibers. The addition of boron carbide to carbon fiber is known to increase fiber strength and, more particularly, to increase the oxidative stability of carbon fibers such that the boron carbide-containing carbon fibers can withstand high temperature environments. Methods of incorporating boron into carbon fibers to form boron carbide fibers have typically involved treating the carbon fibers with gaseous boron halides or impregnation with boric oxides including boric acid, metallic borates and organic borates including alkyl and aryl borates. Upon being treated with the boron compounds, the fibers are heated to initiate reaction of boron with the carbon fibers to yield boron carbide.
For producing an ideal boron carbide fiber, boron levels in the fibers much reach about 78 wt. %. Unfortunately, prior processes for producing boron carbide fibers such as treating carbon fibers with boric acid have typically yielded only small boron loadings, e.g., 2-5 wt. %. Such small boron loadings do not result in any appreciable improvement in the oxidative stability of the loaded carbon fibers at elevated temperatures.
Examples of U.S. patents which disclose incorporating boron into carbon fibers and other shaped articles are discussed below.
U.S. Pat. No. 3,399,979 discloses a method of forming nitride articles by a process which involves impregnating a preformed organic polymeric material with a metal-containing compound, heating the impregnated polymer to carbonize the polymer and heating in an atmosphere containing nitrogen-containing compounds such as ammonia to produce the metal nitride. More specifically, a method is disclosed of immersing a rayon yarn in an aqueous solution of ammonium decaborane and treating the impregnated yarn in ammonia to form boron nitride.
U.S. Pat. No. 3,403,008 discloses a process for producing metal carbide fibers and the like which comprises treating an organic polymeric fiber with a solution containing a metallic compound, heating the metal compound-imbibed polymer to form the carbonaceous fiber and further heating the fiber in a nonoxidizing atmosphere to react the metal with the carbonaceous fiber to form a metal carbide. Among the metal carbides which can be formed is boron carbide obtained by treating the organic fiber with a boric acid solution.
U.S. Pat. No. 3,672,936 discloses providing a reinforced carbon or graphite article having improved oxidation resistance and increased strength by incorporating therein the in-situ reaction product of carbon and a boron containing additive. The process involves making a carbon article such as carbon fiber, dispersing the boron containing additive with at least a portion of the carbon fiber, impregnating the carbon fiber with a carbonizable binder, and heating the fiber to carbonize the binder and to form in-situ the reaction product of carbon and the boron containing additive. Boron containing additives include metal borides, boron nitride, or boron silicides as well as elemental boron.
U.S. Pat. No. 3,971,840 discloses a process of forming a carbide containing fiber of improved strength by a process of heating a carbonaceous fiber in the vapor of a halide of a carbide forming element and heating the fiber under a controlled degree of tension to result in the formation of the carbide. A boron carbide fiber can be formed by treating a carbon fiber with boron trichloride.
U.S. Pat. No. 4,010,233 discloses a method of producing an inorganic fiber comprising a metal oxide phase and finely divided dispersed phase. Boron compounds such as boranes can be used to provide the dispersed phase in the fibers.
U.S. Pat. No. 4,097,294 suggests that a boron carbide ceramic is obtainable from a carborane carbon polymer and that a boron nitride ceramic is obtainable from a borazene polymer. A mixed ternary ceramic is obtained from a polymer with a repeating unit of [C.sub.2 B.sub.10 H.sub.10 R.sub.2 Si(R.sub.2 SiO).sub.n ].sub.x, wherein n is 1 to 10 and x is greater than 4.
U.S. Pat. No. 4,126,652 discloses a method of forming metal carbide articles especially fibers by reacting a metal powder with a carbon fiber such as has been formed from polyacrylonitrile. The fiber may be produced by subjecting a monomer mixture mainly comprising acrylonitrile to solution polymerization with addition of the powdery metal prior to, in the course of, or after the polymerization so as to disperse the metal powder into the polymerization mixture, or alternatively dispersing the powdery metal into a solution of the acrylonitrile polymer in a suitable solvent, and then subjecting the thus obtained powdery metal-containing mixture to molding by a conventional dry or wet method. An example of forming a boron carbide fiber comprises mixing dimethylformamide, non-crystalline boron and polyacrylonitrile to obtain a viscous dispersion and spinning the viscous liquid into fiber which is then heated up to 1800.degree. C. to form the boron carbide.
U.S. Pat. No. 4,197,279 discloses an acrylic carbon fiber with excellent thermal oxidation resistance which contains a phosphorus component and/or a boron component as well as a zinc and/or calcium component. The carbon fibers can be produced by incorporating the above components into the fibers including the acrylic fibers or into the carbon fibers which are produced. For example, the above described compounds can be added to the reaction mixture to produce the acrylic polymer or to a solution of the acrylic polymer before spinning into the fibers. Compounds are added in desired amounts as an aqueous or organic solution or dispersion thereof. Suitable boron compounds which can be used include boric acids, boric acid salts of metals and boric esters such as alkyl borates. The amount of boron included in the fibers is measured in parts per million with amounts as high as only 5100 ppm being described. Similar to this patent is U.S. Pat. No. 4,412,937 which discloses adding 0.01 to 0.03% by weight boron onto a carbon fiber formed from acrylic polymer.
U.S. Pat. No. 4,424,145 discloses a carbon fiber derived from mesophase pitch which has been boronated and intercalated with calcium so that the fiber contains from about 0.1% by weight to about 10% by weight boron and the calcium to boron weight ratio in the fiber is 2:1. The boronating step can be carried out with elemental boron, BCl.sub.3, boranes, or water soluble compounds of boron such as boric acid.
There still exists in the art a need to provide an improved process for producing boron-containing fibers. Thus, there is a need to form boron carbide-containing fibers which have improved oxidation resistance relative to carbon fibers such that the fibers can find increased use in the high temperature, high performance application for which such fibers have vast potential. There is a need to produce improved boron carbide fibers which contain boron in amounts greater than what has heretofore been achieved in the art. Moreover, boron nitride has use as an electrical insulator, a neutron insulator and is corrosion resistant and is, thus, a very useful ceramic which would be advantageous in fiber form.
Accordingly, a principle object of the present invention is to provide boron-containing fiber with increased boron content.
Another object of the invention is to provide boron carbide fibers which have increased oxidative stability at elevated temperatures relative to carbon fibers.
Another object of the present invention is to provide improved boron carbide fibers and an improved process for forming same.
Yet another object of the present invention is to provide improved boron nitride fibers and to an improved process for forming same.
These and other objects and advantages of the invention will be apparent upon consideration of the following description of the embodiments set forth in the description of the invention and the appended claims.